A laser can be used to machine a work piece by removing material or by drilling through the work piece. The laser can provide an unfocused laser beam that has a generally circular transverse cross-section relative to a beam axis A-A with spot size and power (or “irradiance”). The spot size is typically defined as the radial distance from the center of maximum irradiance to a location proximate 0.135 or 1/e2 times the maximum irradiance of the laser beam and measurable with a suitable device. Other techniques can also be used, alone or in conjunction with the previous technique to define the spot size such as, for example, a second moment technique. The spot size can be focused by an optical assembly to a smaller spot size (i.e., a focusing spot size) so that the power of the laser beam can be concentrated over a smaller area, thereby allowing the laser beam to deliver a sufficient power density to the work piece in order to laser machine the work piece.
In laser machining the work piece, the power being applied to the work piece needs to be an appropriate magnitude so that the focusing spot can liquefy, vaporize, and/or ablate the material of the work piece. Where the power being delivered to the focusing spot is much greater than is needed, the focusing spot may cause the work piece to form a heat affected zone (thereby distorting the work piece) or splatter/recasts (thereby affecting the surface finish).
To control the power being delivered to the focusing spot, pulse repetition rate, pulse width, and the laser itself, might be changed. It is commonly believed that a shutter like iris is interposed between the laser beam and the optical focusing assembly. The iris operates to reduce the area of laser light passing through the iris, thereby controlling the power of the focusing spot. The size of the focusing spot in this set up is generally inversely related to the size of the opening of the iris.
As such, whenever the power of the focusing spot size is controlled by changing the beam diameter of laser light passing through the iris, the focusing spot size changes in an inverse manner. For example, when the unfocused beam diameter is reduced through the iris, the focusing spot size increases. The increases in focusing spot size can therefore change the resolution at which the focusing spot size can machine the work piece. Furthermore, when the focusing spot size is changed, the depth of focus of the focused beam changes, which could require the work piece to be repositioned so that the work piece is within this depth of focus for consistent laser machining results. In order to maintain the same focusing spot size or depth of focus, it is believed that the focal length of the focusing lens could be changed to a shorter focal length lens. However, changing the focusing lens could require additional set up time that would affect the overall production efficiency of the laser machining process. Furthermore, when the focal length is reduced, lens aberrations tend to increase. Even though lens aberrations may be correctable using multiple-element focusing optics that would, again, add to set up time and potentially the ability to manufacture the work piece efficiently. Thus, there is an interplay between at least the beam diameter, focusing spot size, focal length of a focusing lens, focusing depth, and even the type of material or dimensions of the work piece that must be taken into consideration whenever the laser power is controlled to a magnitude below its maximum power. This interplay is believed to increase the complexity of the laser machining system.
It would be desirable to maintain a constant focusing spot size regardless of the beam diameter or power being delivered to the focusing spot. It would also be desirable to maintain a generally constant depth of focus regardless of the power being delivered to the focusing spot. It is believed that maintaining these parameters generally constant over a range of power being delivered to the focusing spot would reduce the number of parameters to be considered in a laser machining process.